EPS Bearer Modification Procedure

•   Initiated by network to modify an EPS bearer context with specific QOS and TFT.
•   It can also be initiated when UE sends bearer resource allocation request or bearer resource modification  request.
•    It can also be initiated by network to update APN of UE in case of inter system handover.
•   MME will initiate this procedure by sending MODIFY EPS BEARER CONTEXT REQUEST to UE. MME also includes a EPS bearer identity to identify which EPS bearer Context to be modified.

•          UE shall apply the received TFT for all uplink traffic to radio bearers if the modify request contains a TFT that has packet filters.
•          If there are no Guaranteed Bit Rate or Maximum Bit Rate values for uplink or downlink in received modification request UE shall use previous received values.
•          UE may reject the request with following causes

#26 Insufficient Resources
#41 Semantic errors in TFT

#43 Invalid EPS bearer Identity

          Reference  3GPP TS 23.401

Dedicated EPS bearer context activation procedure

• Dedicated EPS bearer context activation procedure is to establish an EPS bearer context with specific QOS and TFT between UE and EPC.
• This procedure is initiated by the network
• UE  can also request for dedicated bearer by sending resource allocation request or modification request
• MME will initiate dedicated EPS bearer context activation by sending ACTIVATE_DEDICATED_EPS_BEARER_CONTEXT_REQUEST

• The dedicated bearer which will be associated with a default bearer will be identified by the linked EPS bearer identity field in dedicated bearer activation request.
• The linked EPS bearer id also indicates which IP address, PDN the dedicated bearer is linked.
• For all uplink traffic flows to radio bearer UE should use the TFT received in bearer activation request.

·         UE may reject the request with causes such as

o   #26 Insufficient resources

o   #43 Invalid EPS bearer identity

o   #41 Invalid TFT operation.

Reference : 3GPP TS 23.401

Default EPS Bearer Context Activation Procedure

Default EPS Bearer Context activation Procedure

•          Default EPS Bearer Context activation Procedure is an ESM procedure initiated by network.
•          This procedure is used to establish a default EPS bearer context between the UE and EPC.
•          Initiated by network in response to PDN connectivity request from UE.
•          The default bearer context activation can be part of attach procedure. If attach procedure fails default bearer context establish also fails.
•          Default EPS bearer context doesn’t have any TFT assigned during activation, but network may anytime assign a TFT after establishment and may modify the TFT or packet filter of this default bearer.
•          Default bearer context activation procedure initiated by MME by sending Active Default EPS Bearer Context Request.
•          In case initial attach if default bearer is activated , the Activate default EPS bearer context request is sent in attach accept message.
•          If Activate default EPS bearer context request is sent in response to PDN connectivity request from UE, MME will start T3485.
•          MME will assign an EPS bearer identity in ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST.
•          MME will retrieve the PTI from PDN CONNECTIVITY request sent in the activate request from UE.
•          UE should verify the PTI (procedure transaction id) received in the message from network with the PTI sent from UE.
•          UE shall send a reject message with one of the following reject causes

o   #26  Insuffiencent Resources

o   #31  Request rejected, Unspecified

o   #95-111 protocol error

•          If UE provided APN in PDN connection request and received ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST UE shall stop T3396 if it is running for APN provided by UE.
•          If the PDN request is for emergency UE should not stop T3396 if it is running.

·         EPS BEARER :

o   Established between UE to P-GW( E-RAB +S8 bearer)

o   E-RAB is established between UE and S-GW(Radio bearer +S1 bearer)

o   S8 bearer is established between S-GW to P-GW

o   Radio bearer is established between UE and eNodeB.

o   S1 bearer is established between eNodeB to S-GW

·          Default Bearer :

o   Established while attach. Per PDN connection one default bearer will be established. Allocates IP address to UE.

o   No QOS, TFT or Guaranteed bit rate.

·         Dedicated Bearer :

o   Dedicated bearer will be linked to a default bearer.

o   QOS can be Guaranteed bit rate or Non-Guaranteed  bit rate.

o   If default bearer is deactivated dedicated bearer will also be deactivated.

 Reference : 3 GPP TS 24.301

Transport of NAS Message

• This procedure is used to carry SMS messages in an encapsulated form between UE and the MME.
• Can be initiated by the UE or the network and can only be used when UE is attached for “EPS services and Non EPS services” or “EPS services with SMS only” and UE in connected mode.

UE initiated:

• To send an SMS UE iniates UPLINK NAS TRANSPORT message including the SMS message in NAS message container IE.

Network Initiated:

• Network sends DOWNLINK NAS TRANSPORT message which will be sent to SMS entity by EMM entity at the UE.

Generic NAS transport message :

• Used to send other type of data, example an LCS application to send LPP message or a location service message.
• Should not send uplink NAS transport message when timer T3346 is running unless

i)                    UE configured to use access class AC11-AC15

ii)                  UE has PDN connection established for emergency services.

EMM Status Message

• EMM status message is used to report errors detected on the received EMM protocol messages.
• This message can be initiated by either UE or MME.
• No state transitions are seen from radio interface on receiving this message.
• Shall trigger in connected mode. 

  

•  If UE receives a EMM or ESM message with with message type not defined for protocol discriminator or not implemented by the receiver, UE will send EMM status message with EMM cause as #97 Message type not existent or not implemented.
•   Other EMM causes which will be sent in EMM status are

          #96 Invalid mandatory information
          #100 conditional IE errors
          #95 semantically incorrect message

GUTI Reallocation Procedure

• GUTI Reallocation procedure is used to allocate GUTI and optionally new TAI list to the UE.
• This procedure is always initiated by MME in EMM-REGISTERED state
• This procedure is performed in chipered mode, normally takes place in conjunction with other mobility management procedure.
• If UE lower layers indicate transmission failure of GUTI REALLOCATION COMPLETE message with TAI change and is not part of new TAI list UE should abort current procedure and starts TAU.
• IF lower layer failure detected at NW before T3450 timer expiry,NW should consider both old and new GUTI and old and new TAI list as valid.
• For paging first UE should try with S-TMSI from old GUTI and try in old and new TAI list, if response received from UE network should restart the GUTI reallocation procedure again
• In no response received network should use S-TMSI from new GUTI and page in new TAI list.
• If UE sends attach, detach or tracking area update request while on going GUTI procedure network should abort current GUTI procedure and proceed with specific EMM procedure. 

  

                               

Figure : GUTI Reallocation Procedure:

GUTI:

                          

• GUTI size is 80 bits and is composed of Globally Unique MME Identity (GUMMEI) which is of 48 bits long and M-TMSI (Temporary Mobile Subscriber Identity)which of 32 bits long.
• GUMMEI : used to identify the MME which allocated this GUTI. Composed of PLMN (MMC MNC) code, MME Group Id and MME code

T3346 - NAS Level Mobility Management Congestion Control

T3346 timer value may be included in Attch Reject , Tracking Area Update (TAU) Reject or Service Request Reject sent by network to UE. 

During EMM signaling congestion detected network will perform NAS level mobility management congestion control.

By sending reject message with cause value #22 Congestion

If the reject message from network is not integrity protected UE starts T3346 with random value.

if it is integrity protected UE uses the value present in reject message.

NAS Mobility Management Congestion Control consists of

a) general NAS level mobility management congestion control

b) Subscribed APN based congestion control

During general NAS level congestion control

Network shall not reject 

i)attach requests for emergency bearer services.

ii) NAS signaling connections with RRC establishment cause as High priority(Access class 11-15).

iii) Requests for CS fall back emergency services

When APN based congestion control is active the network may reject attach requests from UE's with a subscription to this APN.

While assigning T3346 value network send different values for different UE's so that time outs are not synchronized.

Cases when NAS signalling allowed during T3346 running

•  Attach for emergency services can be initiated even if T3346 timer is running.
•  Attach can be trigger in a new PLMN which is not part of EQPLMN list and also not part of forbidden PLMN list.
•  TAU can be initiated in response to paging if UE in attempting to update state.
• Deatch request can be initiated even T3346 is running

Reasons to stop T3346:

Upon reception of paging, UE shall stop T3346  and send service request or TAU request.

If UE switched off while T3346 is running, on switch on UE should restart the timer with value T3346-switchoff(if the value non zero) time,if UE not able to detect switch off time UE should restart with same T3346 value before switch off.

Though all NAS timers shall be stopped,UE should not stop T3346 in Power Saving Mode(PSM)

PLMN selection

At switch on or recovery from loss of coverage UE should select the last registered-PLMN (RPLMN) if exists and start with the PLMN selection operating mode that was used before switch off/loss of coverage
Two different modes of PLMN selection
i)Automatic PLMN Selection Mode
This mode utilizes a list of PLMN's in priority order. The highest priority PLMN which is available and allowable is selected.
Ms selects and attempts registration on other PLMN/access technology combinations, if available and allowable in the following order
1. Either HPLMN  (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available (if EHPLMN list present)
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order 
5. Other PLMN/access technology combinations in order of decreasing signal quality

ii) Manual PLMN Selection Mode
Here the MS indicates to the user which PLMN's are available, Only when the user makes a manual selection does the MS try to obtain service on the PLMN.
MS indicates the list of PLMN's with all supported access technologies, this list includes PLMN's present in the "Forbidden PLMNs" list, "Forbidden PLMNs for GPRS Service" list
and PLMNs which offers services not supported by the MS.
Plmns meeting above criteria are presented in following order.
1. Either HPLMN  (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available, if more EHPLMNs are available display either the highest priority one or all based on the flag on the SIM.
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order 
5. Other PLMN/access technology combinations in order of decreasing signal quality

Once the UE has registered on a PLMN selected by the user, the UE shall not automatically register on a different PLMN unless
i) The new PLMN is indicated as an equivqlent PLMN by the registered PLMN or
ii) The user selects automatic mode

If no such PLMN selected or that PLMN is no longer available, the MS shall attempt to camp on any acceptable cell and enter the limited service state.

Roaming:
If the MS is roaming and in registered on a VPLMN, the MS shall periodically attempt to select HPLMN (if EHPLMN list is empty) or one of its EHPLMNs (if list is available on SIM) or a higher priority PLMN/access technology combinations listed
in "User and Operator controlled PLMN's list.
A value T minutes stored in the SIM which can range from 6 minutes to 8 hours in steps of 6 minutes to perform periodic attempts.
If no value is present in SIM a default value of 60 minutes is used for T.
The attempts to access the HPLMN or an EHPLMN or higher priority PLMN shall be as specified below:
a) The periodic attempts shall only be performed in automatic mode when the MS is roaming, and not while the MS is attached for emergency bearer services or has a PDN connection for emergency bearer services;
b) The MS shall make the first attempt after a period of at least 2 minutes and at most T minutes:
c) The MS shall make the following attempts if the MS is on the VPLMN at time T after the last attempt;
d) Periodic attempts shall only be performed by the MS while in idle mode

PLMN - Public Land Mobile Network
HPLMN - Home PLMN
EHPLMN - Equivalent HPLMN
RPLMN - Registered PLMN


Reference : 3GPP TS 23.122

Device to Device Communication

• Device to Device Communication has been the latest reasearch topic for LTE advanced. Which enables UE's those are in the proximity to discover and communicate with each other.
• Various use cases involved are
• Social gaming
• Commercial advertisements
• Pubic safety purposes
• Network offloading  

• Enormous operator benefits involved using device to device communications such as
• Peak data rates: as the UE's are near by due to which good radio conditions can enables higher modulation schemes.
• Low latency: No need to go through all the network entities involved in data communication.
• Capacity: Due to spectrum re use UE can accommodate more users
• Extended Coverage : UE's in network coverage can provide services to UE's which are present out side of network coverage (Relay operation)
• With the advancement in the technology and social life opened new ways to communication.
• Prose is a feature that enables operator network controlled discovery and communications between UEs that are in proximity.
• It allows UE's in proximity will discover other UE's in their vicinity and start communication directly with less involvement of network.
• Various use cases involved such as location based advertising (Ads by shops to customers passing near by), social networking applications (facebook, twitter),network off loading and Public safety etc.
• The communication path could for example be established directly between the UEs or routed via local eNB(s).

Discovery Types : 

• Open ProSe Discovery: is ProSe Discovery without explicit permission from the UE being discovered.
• Restricted ProSe Discovery: is ProSe Discovery that only takes place with explicit permission from the UE being discovered. UE's can have a feature enabled whether it can be discovered by other UE's are not (privacy).
• Operator can change dynamically the range of the proximity per user/application. 

Disovery Models :

Model A:

• Model A is push model like "i am here". used for both open and restricted discovery.

Model B:

• Model B is pull model like "who are there" or "Are you there". it is used for restricted discovery. 

3GPP TR 22.803,3GPP TS 23.303

T3412 - Periodic Tracking Area Update timer

Periodic Tracking Area Update procedure to used to periodically notify the availability of the UE to network.

UE needs to notify its presence to the Network periodically and is controlled using T3412 timer in LTE.

One of the reasons for this is that at any time network will be aware of the UE's presence at a granularity of Tracking Area. Which will be needed for cases such as paging.

Default value of this timer is 54 min.

The value of this timer is provided by the network operator during the attach and tracking area updating procedures

If this value is not present or value is zero (0) UE should not trigger periodic tracking area procedure.

T3412 contains two parameters

i)timer_unit

   If timer unit is 2 it represents that timer value is represented in deci hours.

ii)timer value

    If timer value is 1 it means 6 mins (if timer unit is 2)

T3412 will be stopped when UE enters EMM-CONNECTED or EMM-DEREGISTERED mode.

T3412 will be reset and started with initial value when UE moves from EMM-CONNECTED to EMM-IDLE mode.

When the network includes T3412 extended value IE in the ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message, the network uses timer T3412 extended value IE as the value of timer T3412

On expiry of the timer UE initiates Periodic TAU Request if the UE is not registered for the emergency bearer services or T3423 running.Implicit detach from network if the UE is attached for emergency bearer services.

Network will maintain a mobile reachable timer which is 4minutes greater than T3412. If UE not updated TAU with in mobile reachable timer network shall implicitly detach the UE

According to 3GPP 24.301